1. Field of the Invention
This patent specification relates to chromatography. More particularly, this patent specification relates to systems and methods for chromatography under high-pressure and/or high temperature environments.
2. Background of the Invention
Chromatography is the field of separating chemicals based on differences in properties such as volatility, absorption, adsorption, size, etc. In this field, different rates of migration along a given flow path (gas, liquid, supercritical fluid, etc) result in the spatial separation of chemical analytes. This differential migration is achieved by differing rates of interaction with the separation column or by different values of analyte mobility.
Typical gas chromatography (GC) separation columns are small diameter tubes that can be more than 15 meters long. The column, usually wound in a coil, is housed inside a heated compartment or co-wound with heater wires. The heater is designed to keep the column at either constant temperature, or for certain analyses, provide it with an increasing and fast temperature ramp. In the case of a ramped system, after a sample analysis is completed, the column must be cooled to the lower starting temperature. The cooling process can be particularly time consuming unless means of cooling such as fan, thermal switch, etc. is provided. However, the heating and cooling apparatus contributes significantly to the total mass, which makes the heating and especially the cooling function slow and inefficient.
Certain environments, such as oil and gas wells, have unusually harsh ambient conditions. For example, it is not uncommon for the ambient conditions within the borehole to be greater than about 100° C. and greater than about 100 atmospheres in pressure. In order to operate a chromatograph under such conditions, existing methods would require depressurization of sampled fluid to around 1 atmosphere, and cooling of certain components of the chromatography apparatus considerably below the ambient temperature. Such methods pose significant challenges for implementing chromatography equipment in downhole environment. Additionally, for the high temperature downhole environment, it is difficult to reject heat from the column/heater/cooler apparatus.
Samples of downhole fluids such as collected using a downhole sampling tool or collected on the surface very close to the wellbore are typically stored in bottles and shipped to laboratories for analysis. This transportation process to the laboratory takes a significant amount of time which can be very costly for some applications such as off shore operations. Additionally, the sample at the laboratory, in order to be analyzed using convention chromatography systems, needs to be depressurized. The process is sometimes complex and during transportation, handing and preparation the pressure and temperature changes can induce significant changes in the chemical properties of the sample prior to analysis.
Some research has been published on the relationship of high pressures to the retention times in chromatography. For example, T. L. Kwa, High-pressure gas chromatography: I. A precision high-pressure gas chromatograph for isobaric-isothermal measurements, Journal of Chromatography A, Vol. 270, 1983, P. 105-115 discloses a high pressure chromatograph with a packed column used to investigate thermodynamic and transport properties of certain fluids. In another example, Viktor B. Berezkin, Alexander A. Korolev, and Irina V. Malyukova, Pressure effect on Relative Retention in Capillary Gas-Liquid Chromatography, J. High Resol. Chromatogr., 1997, Vol. 20, June, P. 333-336 discloses experiments up to about 10 atmospheres to understand the dependence of relative retention and retention indexes on average column pressure. However, these prior systems and methods were primarily concerned with understanding pressure relationships with retention time and did not attempt to propose any techniques for optimizing resolution at high pressures.